Silicon deposition from silane inside a fluidized bed reactor is an endothermic reaction that requires an external energy source. Maintaining a desired operating temperature in such a reactor is difficult.
Heat can be added through the reactor walls. Alternatively, reactant streams can be preheated before they enter the reactor. In some cases, energy also can be “beamed in” via electromagnetic radiation, such as microwaves or lasers. Each of these methods has certain limitations.
The microwave option requires a suitable generator connected to the reactor. The generator must be able to withstand the temperatures and pressures of the reaction. The reactants must be able to absorb the wavelength of radiation the generator produces while the products must not absorb that same radiation lest the reaction be reversed back to the feedstocks. The generator also must operate at a reasonable efficiency. These conditions have seriously limited the use of electromagnetic radiation as the primary energy source in fluidized bed reactor designs.
Heating the reactor through the walls is simple and efficient. However, high heat fluxes require high temperatures which can seriously degrade the strength of the walls. Increasing the thickness of the walls to compensate for the lower strength reduces heat flow, which requires even higher temperatures to drive the heat through the walls. The higher temperatures weaken the walls further, which sets up a vicious cycle that soon reaches the point of diminishing returns. Also, the cross-sectional area of the reactor increases as the square of the reactor's diameter, while the available area for heat transfer (the circumference of the reactor) increases linearly with diameter. Thus, reactor size is limited by the heat transfer capacity.
Heating the reactants before they enter the reactor eliminates the need to transfer heat through the reactor walls. However, if the reactants are too hot, there is a risk of inducing an undesirable reaction in the feed piping. Inside the reactor, the entire fluidized bed should remain at the desired temperature. When performing an endothermic reaction, the reactants are superheated before entering the reactor to maintain the desired temperature throughout the entire bed. The amount of superheating may be so high that the bottom of the bed (where the reactants enter the fluidized bed reactor) is far above the desired operating temperature. This excessive bottom temperature can cause unwanted side reactions, or can cause the reactants to react immediately upon reaching the reactor instead of throughout the bed volume as intended. In either case, low yields or plugging at the inlet ports may result.
An independent issue in many fluidized bed reactors is the formation of large bubbles within the bed in the main reaction zone under certain conditions. Large bubbles are usually undesirable for several reasons.
First, the best heat and mass transport occurs with small bubbles. The reactor functions by exchanging reactants, products, and energy between the bed, made of a large number of small particles, and the fluid phase. Heat and mass transfer occurs at the surface of a bubble. As the mass and energy transport is often the limiting factor in determining the production rate, faster transport is desirable. Two ways to improve the transport rate include having a thinner boundary layer in the bubble and increasing the surface area available for transport. Unfortunately, the bubble's boundary layer is set by the reactor conditions and is not easily modified. However, with skill, the surface area can be greatly enlarged by designing the fluidized bed reactor system to have many small bubbles instead of a few large ones. One way to create smaller bubbles is to install a mechanical device called a bubble breaker. This device disrupts the large bubbles, breaking them up into multiple smaller bubbles and promoting better mixing.
Another unwanted effect of large bubbles, particularly in gas-solid systems, is that they cause the bed to bounce violently up and down as they lift a significant fraction of the bed, then drop it suddenly. This pressure oscillation can interfere with proper operation of the bed by causing the gas velocity rate to vary, which may be harmful to optimum productivity. The pressure oscillation also causes mechanical stress to the reactor structure and any directly connected support equipment. Again, using a bubble breaker to reduce the bubble size reduces or eliminates the unwanted effects of large bubbles.
Bubble breakers typically are static objects, such as coarse mesh screens or grids of bars or pipes that are installed across the direction of flow. Traditionally their role is solely to break up the large bubbles, producing several small bubbles in their place, and to not otherwise take part in the reaction within the reactor. Apparatus and methods for chemical vapor deposition on seed particles inside a fluidized bed reactor are disclosed herein. Improved techniques for converting silicon-bearing gasses to polycrystalline silicon in a solid form also are described.
In described arrangements, an obstructing member or bubble breaker is provided within the fluidized bed reactor. The bubble breaker is constructed to supply additional surface area for the addition of heat to the reaction zone. This effect can be used to reduce wall temperatures of the reactor, increase the diameter of the reactor, or both.
One or more pipes or similar conduits are positioned within a reactor to provide a bubble-breaker at a suitable location for the disruption of bubbles. Gas that has been heated above the desired reaction temperature flows through a passageway within the bubble breaker. Heat is transferred from the gas to the reaction zone. The cooled gas exiting from the bubble breaker can be pumped in a closed circuit back to its heater, sent to the reactor inlet at its exit temperature, or reheated and sent to the reactor inlet at some other temperature.
In particular arrangements, the bubble breaker includes two, three, four, or more generally U-shaped pipes or tubes. Flow through the individual tubes can be in series or parallel.